Switching power converters are useful for converting electrical power having certain characteristics into electrical power having one or more different characteristics. For example, switching power converters can convert a first type of power having a particular voltage or current level or frequency of operation into a second type of power having a different voltage or current level or different frequency of operation. Switching power converters can include, for example, AC to DC converters (e.g., rectifiers), DC to AC converters (e.g., inverters), DC to DC converters (e.g., buck converters, boost converters, and buck-boost converters), and AC to AC converters. Also for example, switching power converters can be single-phase power converters, or multi-phase (e.g., three-phase) power converters. Due to their effectiveness and versatility, switching power converters have become ubiquitous in a variety of applications including residential, industrial and military applications. Their application is further slated to grow in automotive and aerospace systems, for example, in connection with traction-related applications and various accessories.
Despite their wide use, during operation many switching power converters have the undesirable side-effect of generating electromagnetic interference (EMI). This side-effect is particularly disadvantageous insofar as, in recent years, the concern over EMI occurring within and between various types of equipment has become heightened, which in turn has lead to the adoption of various regulatory regimes concerning EMI and electromagnetic compatibility of electrical devices. EMI generated by switching power converters can be particularly significant due to the high frequency switching of electrical quantities (e.g., high frequency changes in voltage and/or current), and due to the short transition intervals involved. The high frequency signal components tend to find parasitic capacitive coupling paths through various circuit elements. In particular, common mode current paths formed by way of the components of switching power converters and parasitic capacitances existing between the converters and ground often constitute a source of significant capacitive coupling and conducted EMI.
In order to mitigate the amount of EMI generated by switching power converters, some conventional switching power converters include or operate in conjunction with passive common mode filters, which can be formed, for example, by way of multiple stages of L-C filters having appropriate damping. These filters commonly are aimed at curbing conducted electromagnetic emissions in the 150 kHz to 2 MHz frequency band, and are designed to provide adequate attenuation across this frequency band to reduce the conducted noise caused by power semiconductor switching devices employed within the switching power converters. While not entirely eliminating EMI, such filters are somewhat effective for use in conventional switching power converters insofar as such power converters often employ power semiconductor switching devices that have an emission spectrum that drops off at 20 dB/decade beyond the switching frequency (Fs) of the converter and at 40 dB/decade beyond the transition frequency (π/tr), where tr is the switching transition interval of the switching devices.
Notwithstanding the limited effectiveness of conventional passive common mode filters in reducing the EMI generated by conventional switching power converters, such filters are likely to be less effective in the future as the switching power converter industry moves toward power converters with higher power densities and performance levels that require higher switching frequencies and reduced transition intervals. Higher power densities generally result in higher levels of EMI. Additionally, as the switching frequency of a power converter increases, larger energy levels of noise become present at the low frequency end of the band of interest. Further, as the switching transition interval tr is reduced, and the transition frequency concomitantly increases, the higher noise level must decay (e.g., at the rate of 20 dB/decade) over a broader range within the frequency range before reaching the transition frequency.
The limitations of conventional passive common mode filters in this regard can be seen in particular by considering an exemplary conventional (Prior Art) passive second order EMI filter 2 as shown in FIG. 1. As shown, the filter 2 is implemented in relation to a power source 4 and a switching power converter 6, which in turn is also coupled to a load 8. The filter 2, power source 4, switching power converter 6, and load 8 overall can be considered to form a system 0. The filter 2 includes an inductor 3 and first and second capacitors 5 and 7, respectively, which are coupled between first and second input terminals 10 and 12, respectively, of the power converter 6 and first and second output terminals 14 and 16, respectively, of the power source 4. More particularly, first and second magnetically-coupled coils 9 and 11, respectively, of the inductor 3 are respectively coupled in series between the first and second output terminals 14 and 16, respectively, and the first and second input terminals 10 and 12, respectively. Additionally, the capacitors 5 and 7 are coupled in parallel within one another between the input terminals 10, 12. The capacitor 5 provides a common mode capacitance and the capacitor 7 provides a differential mode (or normal mode) capacitance, while the inductor 3 provides a common mode inductance. Differential mode inductance occurs in practice only as a result of imperfection and parasitic effects and, ideally, the inductor 3 provides no differential mode inductance. In the exemplary embodiment shown, the capacitor 5 is shown to be broken into two parts 13 and 15, which are coupled in series with one another, and where a node coupling the two is coupled to ground so as to allow for common mode filtering.
Further, in the exemplary embodiment shown, the switching power converter 6 is a buck switching power converter having a first capacitor 17 coupled between the first and second input terminals 10 and 12 and a transistor 19 acting as a switching device, the collector of which is also coupled to the first input terminal 10. The buck switching power converter 6 further includes an inductor 21 coupled between the emitter of the transistor 19 and a first output terminal 18 of the power converter. Additionally, the power converter 6 includes a second capacitor 23 coupled between the first output terminal 18 of the power converter and a second output terminal 25 of the power converter (which are coupled to the load 8) and a diode 27, the cathode of which is coupled to the emitter of the transistor 19 and the anode of which is also coupled to the second output terminal 25 of the power converter, which is the same node as the second input terminal 12. Further, the buck switching power converter 6 is shown to include a parasitic capacitor 29 that links the emitter of the transistor 19 to ground. It is by way of the parasitic capacitor 29 that some of the common mode currents are able to flow, resulting in the generation of EMI by the power converter. Although FIG. 1 shows the power converter 6 to be a buck converter, such a converter is only shown as one example of a variety of different power converters.
Referring again to the filter 2, in combination, the common mode inductance and common mode capacitance form a second order filter to provide common mode noise mitigation, while the differential mode capacitance along with any nonideal differential mode inductance that may be present form a second order filter to provide effective differential mode noise mitigation. To increase the differential mode attenuation, the capacitance value of the capacitor 7 can be increased to any (or almost any) arbitrary value. Yet the same is not true for the purpose of increasing the common mode attenuation. Rather, because the largest capacitance value of the capacitor 5 is bounded by ground leakage current considerations (as well as, in possibly some circumstances, safety considerations), it is not always possible to use an arbitrarily large capacitor. Further, although a desired level of attenuation could also be obtained by selecting an arbitrarily large inductance value for the inductor 3 (so as to achieve a larger common mode inductance), this is often not possible insofar as large inductors dissipate more heat/power, are physically large, and cannot be easily implemented on integrated circuits.
Given the limitations of conventional passive filters, particularly in terms of their ability to filter common mode currents, efforts have been made to develop other circuits or methodologies for reducing common mode currents. Among these have been circuits that replace the passive filters with active filters or hybrid filters having both passive and active elements. Typically, some such hybrid filters include electronic circuits that are designed to inject noise currents of appropriate magnitude and phase so as to cancel out parasitic common mode noise currents.
Although conventional hybrid filters have some limited effectiveness, conventional hybrid filters are unsatisfactory in their performance. Generating currents that perfectly or substantially cancel out parasitic common mode noise currents is difficult to perform, and becomes even more important as the bandwidth over which those parasitic common mode noise currents occur becomes large. Complicating matters, the operation of conventional hybrid filters often is not easy to model and consequently is not easy to predict or guarantee. Thus, conventional hybrid filters not only are unable to fully achieve the goal of canceling out common mode noise currents, but also it is difficult or impossible to determine why this is so and to determine how the filters can be modified to achieve enhanced performance.
For at least the above-described reasons, therefore, it would be advantageous if an improved device or system could be developed, for implementation as part of or in conjunction with switching power converters, that served to better reduce the EMI generated by such switching power converters, particularly the EMI generated as a result of common mode currents of those power converters. Further, it would be advantageous if in at least some embodiments such an improved device could successfully reduce the EMI of switching power converters that operate at higher power densities, at higher switching frequencies and/or with reduced transition intervals.